Neutrophils (PMN) are established mediators of immune renal injury. Our objective is to test a new hypothesis to explain how PMNs cause renal damage. New evidence strongly suggests that PMN-derived reactive oxygen species (ROS), particularly H2O2, are the major mediators of tissue injury. However, how H2O2 in physiologic concentration causes tissue injury is not known. We hypothesize that the cationic PMN enzyme myeloperoxidase (MPO) binds on a charge basis to the glomerular capillary wall and catalyzes the reaction of H2O2 with a halide (C1, I) to form highly toxic hypohalous acids and halogens. We have now shown that MPO and H2O2 can interact to produce severe glomerular injury and that a similar mechanism may be operative in a model of immune complex nephritis. We now wish to expand these preliminary observations in three areas. First, we will establish the importance of the MPO system in PMN-mediated injury by studying glomerular injury in PMN- depleted rats with immune complex nephritis (GN) that are reconstituted with normal or MPO deficient human PMNs. Second, we will study the effects of the MPO-H2O2-halide system in vitro on resident glomerular cells and glomerular basement membrane (GBM) which may account for its nephritogenicity. This would include examining the ability of the system to cause hydroxyproline release from GBM collagen and the lethal and sublethal effects of the MPO system on the mesangial cell. Finally, we will pursue some interesting preliminary data which suggests that platelets may play an important role in PMN/MPO mediated GN by studying rats depleted of platelets and by performing kinetic studies with isotopic labelled platelets and PMNs in PMN-mediated immune complex GN.